1. Field of the Invention
The present invention relates, in general, to devices for conducting electrical signals between bodies undergoing relative rotational motion and, more specifically, to electrical slip rings.
2. Description of the Art
Electrical slip rings are widely used to conduct electrical signals between a first stationary body and a second body undergoing rotation relative to the first body. A typical slip ring includes a stator and a rotor rotatably connected thereto. A number of brushes and contact rings are mounted on the stator and rotor and are disposed in continuous sliding contact, unless a brush lifter mechanism which separates the brushes from the contact rings is activated. One to several brushes may be in contact with each contact ring. Either the brushes or the contact rings may be mounted on the rotor; while the mating contact ring or brush is mounted on the stator.
Slip rings have been used to provide a connection path for a variety of sensors, such as strain gages, thermocouples, pressure transducers, resistance temperature devices (RTDs), torque transducers, accelerometers, velocity sensors, etc., mounted on a rotating member, such as a train axle or wheel, an automobile wheel, etc., to a data acquisition unit located on the train or vehicle remote from the sensor. In instrumentation systems involving a large number of sensors, the number of brushes and contact ring pairs in the slip ring becomes extensive since each signal requires a separate conductive path, i.e., a separate brush/contact ring pair, through the slip ring. Although some common connections of power and ground leads can be implemented, such does not significantly reduce the number of brush/contact pairs required in a slip ring. The result is a large and therefor expensive slip ring. This is a detrimental factor, not only from a cost standpoint, but also in certain applications where space is at a minimum. In such applications, a large slip ring cannot be used.
Another factor encountered in such sensor based instrumentation system is the introduction of false signals or noise onto the sensor output. The use of such instrumentation systems on moving vehicles, trains, etc., exposes the extremely low level signals from the sensors to random external electrical signal generating sources, such as RFI and EMI generators, etc. These devices generate electrical and magnetic fields and electrical signals which can cause false signals or noise within the sensor output signal which introduces errors into the recorded data.
Further, in certain applications, such as instrumentation systems employed on trains, the sensors are located a considerable distance from the data acquisition unit. This requires long cables or conductor runs between the sensors and the data acquisition unit. Such conductors or cables are sensitive to temperature which effects their resistance and, further, the long cable lengths can introduce a voltage drop in the signals due to the resistance of the conductors. This again effects the accuracy of the data received by the data acquisition unit.
Thus, it would be desirable to provide a slip ring for use in a rotating electrical signal generating data acquisition apparatus which overcomes the aforementioned problems relating to the use and construction of slip rings. It would also be desirable to provide a data acquisition apparatus which minimizes the introduction of erroneous electrical signals caused by RFI, EMI and other sources. It would also be desirable to provide such an apparatus which has a small size for easy mounting on an existing rotatable member undergoing testing. It would also be desirable to provide such an apparatus which minimizes noise and cable losses between the sensors and a remote data acquisition unit. It would also be desirable to provide such an apparatus which eliminates the need for costly cable requirements between the sensors and the data acquisition unit. Finally, it would be desirable to provide such an apparatus which is constructed to reduce the number of separate number of individual sensors.